One characteristic required of electrical steel sheets is the magnetic flux density under a low magnetic field. As for non-oriented electrical steel sheets used as iron cores of motors, this characteristic is an important factor governing the efficiency of motors.
In general, the magnetic properties of the electrical steel sheet under the low magnetic field depend on the movability of the domain walls, and are mainly effected by micro-structures such as grain boundaries, fine precipitates, non-metallic inclusions, lattice defects or internal stresses.
Among them, the grain boundaries (grain diameter), fine precipitates and non-metallic inclusions are preliminarily controlled by birthes of steel themselves, and the lattice defects (strain) and the internal stress are very often introduced by external factors during final annealing.
With respect to external strain factors negatively influencing the magnetic properties under the low magnetic field, the most important factors in processing are strains which are caused by tension in an annealing line, bending deformation by the rolls in a furnace or thermal stress during cooling.
There recently has been heavy demand for thin gauge electrical steel sheets, aiming at low iron loss. In view of the objective of keeping the flatness of the steel sheet and improving its properties under the low magnetic field, slow coolings are indispensable within ranges improving the tension and precision without decreasing the productivity. The method for cooling in a final annealing, taking the magnetic properties into consideration, has been proposed in Japanese Patent Laid-Open Specification No. 96,919/77. This proposal specifies the cooling rate from the soaking temperature to 300.degree. C. at not more than 250.degree. C./min for decreasing the iron loss. However in the annealing of 1000.degree. C. shown in the Example, this technique takes 2.8 minutes for cooling from 1000.degree. to 300.degree. C., and uses a long cooling zone. If the running speed of the strip is made slow, not only does the productivity go down, but also it takes a long time for annealing, so that the magnetic properties (especially iron loss) are sometimes deteriorated reversely by extraordinary grain growth during soaking at the annealing temperature.